MOSSBAUER SPECTROSCOPIC STUDIES ON THE IRON FORMS OF DEEP-SEA SEDIMENTS

Mossbauer spectroscopy was applied to characterize the valence states Fe(II) and Fe (III) in sedimentary minerals from a core of the Peru Ba sin. The procedure in unraveling this information includes temperature -dependent measurements from 275 K to very low temperature (300 mK) in zero-field and also at 4.2 K in an applied field (up to 6.2 T) and by mathematical proce dures (least-squares fits and spectral simulations ) in order to resolve individual spectral components. The depth distri bution of the amount of Fe(II) is about 11% of the total Fe to a depth of 19 cm with a subsequent steep increase (within 3 cm) to about 37%, after which it remains constant to the lower end of the sediment core (at about 40 cm). The steep increase of the amount of Fe (II) defines a redox boundary which coincides with the position where the tan/gree n color transition of the sediment occurs. The isomer shifts and quadr upole splittings of Fe(II) and Fe(III) in the sediment are consistent with hexacoordination by oxygen or hydroxide ligands as in oxide and s ilicate minerals. Goethite and traces of hematite are observed only ab ove the redox boundary, with a linear gradient extending from about 20 % of the total Fe close to the sediment surface to about zero at the r edox boundary. The superparamagnetic relaxation behavior allows to est imate the order of magnitude for the size of the largest goethite and hematite particles within the particle-site distribution, e.g. similar to 170 Angstrom and similar to 50 Angstrom, respectively. The composi tion of the sediment spectra recorded at 300 mK in zero-field, apart f rom the contributions due to goethite and hematite, resembles that of the sheet silicates smectite, illite and chlorite, which have been ide ntified as major constituents of the sediment in the <2 mu m fraction by X-ray diffraction. The specific ''ferromagnetic'' type of magnetic ordering in the sediment, as detected at 4.2 K in an applied field, al so resembles that observed in sheet silicates and indicates that both Fe(II) and Fe(III) are involved in magnetic ordering. This ''ferromagn etic'' behavior is probably due to the double-exchange mechanism known from other mixed-valence Fe(II)-Fe(III) systems. A significant part o f the clay-mineral iron is redox sensitive. It is proposed that the co lor change of the sediment at the redox boundary from tan to green is related to the increase of Fe (II)- Fe(III) pairs in the layer silicat es, because of the intervalence electron transfer bands which are caus ed by such pairs.